There has been known a magnetic fluid seal that seals an annular gap between a shaft and a shaft hole of a housing with magnetic fluid retained by a magnetic force (see Patent Literature 1). The magnetic fluid seal disclosed in Patent Literature 1 is provided with an annular permanent magnet having magnetic poles at axial direction both ends, and a pair of magnetic pole members provided on axial direction both sides across the permanent magnet and retaining the magnetic fluid between the magnetic pole members and the shaft. With such a configuration, a magnetic circuit is formed to pass through the permanent magnet, the pair of magnetic pole members, and the shaft made of a magnetic body. Therefore, the magnetic fluid is stably retained between the shaft and the magnetic pole members.
A function and a conventional assembly method of the magnetic fluid seal having the configuration explained above are explained with reference to FIG. 5. FIG. 5 is a schematic partial view showing a state after components of a magnetic fluid seal according to a conventional example are attached to a shaft. Only components necessary for explanation are illustrated. Note that an upward direction in FIG. 5 is a vertical upward side. A magnetic fluid seal 900 according to the conventional example includes a first magnetic pole member 902 and a second magnetic pole member 903, both of which are annular, on axial direction both sides of an annular permanent magnet 901 (hereinafter simply referred to as “magnet 901”) divided in the circumferential direction and further includes a first bearing 904 and a second bearing 905 on both sides of both the magnetic pole members 902 and 903. A plurality of annular protrusions 402 are provided in the annular groove 401 provided on the outer circumferential surface of a shaft 400. Note that the plurality of annular protrusions 402 are provided to be divided into a first annular protrusion group 411 and a second annular protrusion group 412. In the magnetic fluid seal 900 configured in this way, magnetic fluid (not shown in the figure) is retained in annular gaps between the outer circumferential surfaces of the annular protrusions 402 and the inner circumferential surfaces of both the magnetic pole members 902 and 903 by a magnetic force acting from a magnetic circuit formed by the magnet 901.
In a conventional assembly method of the magnetic fluid seal 900, first, the first bearing 904 is fixed in a predetermined position of the shaft 400 that is in a state in which the shaft 400 is directed in the vertical direction. Subsequently, after the magnetic fluid (not shown in the figure) is applied to the vicinity of the first annular protrusion group 411, the first magnetic pole member 902 is placed on the first bearing 904. Subsequently, an annular jig (not shown in the figure) having a shape same as the shape of the magnet 901 and divided in the circumferential direction is placed on the first magnetic pole member 902. Subsequently, after the magnetic fluid is applied to the vicinity of the second annular protrusion group 412, the second magnetic pole member 903 is placed on the annular jig. Both the magnetic pole members 902 and 903 are in a state in which the magnetic pole members 902 and 903 are positioned with respect to the shaft 400. However, the magnetic pole members 902 and 903 are only stacked on the other components and are not particularly fixed. Divided pieces of the divided jig are replaced with divided pieces of the divided magnet 901 one by one, whereby attachment of the magnet 901 is performed in a state in which the second magnetic pole member 903 is placed on the jig. When the attachment of the magnet 901 is done, the second bearing 905 is attached from the upward direction and fixed in a position where the second bearing 905 is in contact with the second magnetic pole member 903. Consequently, both the magnetic pole members 902 and 903 and the magnet 901 are fixed in the state in which the magnetic pole members 902 and 903 and the magnet 901 are positioned with respect to the shaft 400.
As explained above, in the conventional assembly method of the magnetic fluid seal 900, the magnet 901 is attached before both the magnetic pole members 902 and 903 are fixed by the second bearing 905. Therefore, in the conventional assembly method, in a period from after attachment of the divided pieces of the magnet 901 until before attachment of the second bearing 905, the first magnetic pole member 902 and the second magnetic pole member 903 are sometimes attracted to the shaft 400 by the magnetic circuit formed by the magnet 901. When the magnetic pole members move in this way, there is a concern about deterioration in sealing function of the magnetic fluid seal 900. When a movement amount is large, it is likely that the moved magnetic pole members come into contact with the shaft 400. Therefore, the movement of the magnetic pole members is undesirable.
It is conceivable to adopt a method of fixing both the magnetic pole members 902 and 903 by attaching the second bearing 905 before replacing the divided pieces of the jig with the divided pieces of the magnet 901. However, when the second bearing 905 is fixed first, it is difficult to take out the divided pieces of the jig radially outward because the divided pieces are pressed by both the magnetic pole members 902 and 903. When the divided pieces of the jig are taken out, the gap between both the magnetic pole members 902 and 903 is narrowed. Therefore, attachment of the divided pieces of the magnet 901 could be difficult. Therefore, the method of fixing the second bearing 905 first is unrealistic.
As explained above, when the magnetic fluid seal provided with the magnetic pole members (holding portions for the magnetic fluid) on both the sides of the magnet is assembled by the conventional method, the magnetic pole members sometimes move from the positions where the magnetic pole members are positioned due to a magnetic force from the attached magnet.